Translocation Of Mineral Ions
Most mineral ions are absorbed by roots through active or passive processes, or a combination of the two. They are further transported up to the xylem, and from there they reach all over the plant body. Mineral ions are frequently remobilized, particularly from older, senescing parts to the young growing tissues.
Common minerals that are remobilized in plants are P, S, N, and K. Minerals such as Ca are not remobilized. This translocation of minerals takes place by two paths. They are
Radial translocation in root: Solutes absorbed by root hair pass inwardly through the epidermis, cortex, endodermis, pericycle, xylem parenchyma, and xylem channels. Translocation of mineral ions takes place through apoplasts, symplast, or both.
Upward translocation in shoot: This takes place through xylem channels along with the ascent of sap. Hoagland and Stout (1939) confirmed that the xylem takes most of the absorbed minerals along with the water current to leaves.
mechanism of nutrient uptake in plants
Minerals are carried by the phloem if they are needed to be distributed from leaves to other parts of the plant body. Minerals convert to organic form and can diffuse into the xylem and are carried upward.
The chief storage regions or sinks for the mineral elements are meristems, young leaves, developing flowers, fruits, seeds, and storage organs. Minerals are unloaded into the living cells from the fine vein endings through diffusion and active uptake.
Phloem Transport – Flow From Source To Sonk
The carbohydrates synthesized through photosynthesis, are transported from the leaves to different parts of the plant by the phloem tissue. This food is utilized by the plant for cellular respiration and for storing. The leaf acts as the source and the utilizing organs and the storage organs are the sinks.
Plant hormones or phytohormones are synthesized by different parts of the plant such as roots, stems, leaves, etc. These hormones are also transported by phloem tissue.
” nutrient uptake in plants”
Translocation Of Organic Solutes
Definition: The process by which organic food, other organic compounds, and hormones are transported through phloem tissue to the main storage organs or different sites of action of plants, is called translocation of organic solutes.
Green plants containing chlorophyll in their leaves synthesize food in the presence of sunlight. Glyceraldehyde-3-phosphate (GAP) is the primary product of the Calvin Cycle. It is used in a variety of biosynthetic pathways, both inside and outside the chloroplast.
For the first time, it is transported out of the chloroplast of the source cells (mesophylls) in the form of dihydroxyacetone phosphate (DHAP). The triose-phosphates combine in the cytosol to form fructose 1,6-bisphosphate (F6P) by the enzyme aldolase.
F6P is then isomerized to glucose 6-phosphate by the enzyme hexose phosphate isomerase. The glucose and fructose molecules are ultimately converted to sucrose for transport.
Sucrose is then loaded in the phloem sieve tube through apoplastic (cell wall) or symplastic (through plasmodesmata) pathways for long-distance transport. Sucrose is transported to the site of utilization or storage or sink.
root microbe interaction in hindi
Plant hormones or phytohormones are synthesized by different parts of the plant such as roots, stems, leaves, etc. These hormones are also transported by phloem tissue.
Plant tissues involved in the translocation of solutes The Translocation of solutes in plants occurs through phloem tissue.
Plant tissues involved in the translocation of solutes
Translocation of solute in plants occurs through phloem tissue. Sieve tubes of phloem are the main components that translocate solute. Companion cells remain associated with each sieve tube and accompany sieve tubes in this process. However, according to modern scientists (Cutter, 1978), companion cells too translocate solute.
Cells of sieve tubes (sieve cells) are without a nucleus and stop;asm is present in the form of a primordial utricle. These cells are elongated and cylindrical in shape. Sieve cells are arranged consecutively along their length to form sieve tubes.
plant nutrient uptake mechanisms
The sieve tubes are so named because their end walls are perforated. These perforations allow cytoplasmic connections (plasmodesmata) between the cells. This helps in the transport of sugars and amino acids from the leaves, to the rest of the plant. Companion cell provides energy to the sieve cells.
Companion cells transport products of photosynthesis from cells of leaves to the sieve tube elements through plasmodesmata. They synthesize the various proteins used in the phloem. They also contain numerous mitochondria that provide the energy required for active transport.
Solute transport pathway: Translocation of solute occurs mainly in a descending manner. However, other modes of translocation like, ascending and lateral translocation also occur to some extent.
Descending translocation: The food produced in the leaves and other photosynthetic parts of plants, is translocated downward in a dissolved state through phloem tissue to roots, underground stem, and other storage parts of the plant. Some hormones are produced at the tip of leaves, stems, and branches which are transported downward to their target organs.
Ascending translocation: The organic food, solvent, and different phytohormones are also translocated upward in plants. During winter, many deciduous plants shed their leaves. In such plants, the stored food is dissociated into simple sugar which is transported through phloem to different parts of plants.
Lateral translocation: According to modern concepts, translocation of solute also occurs laterally. Trees show lateral growth of branches and stems. These lateral branches of roots and stems carry out lateral transport of solute.
Translocation of starch through the phloem
There is experimental proof that the transport of organic food occurs through phloem tissue.
Ringing or girdling experiment:
This experiment was done by T. G. Mason and E. J. Maskell (1928). They took a potted woody plant and carefully cut out a ring in the woody stem of the plant. The ring is cut outside the vascular cambium in such a way that only the bark, cortex, and phloem are removed.
“mineral absorption “
Thus, the upper region of the plant remains connected with the lower region, through the xylem and pith only. After a few days, they observed that the stem of the upper part at the cut region had swelled up and adventitious roots had also developed. It has also been observed that the underground roots degenerate after some days.
This experiment proved that the translocation of food through phloem is descending. When the phloem is cut out, food accumulates at the cut region and cannot reach up to the roots. Due to the absence of food, roots ultimately die.
Isotopic study: Burr et al (1945) used radioactive carbon to trace descending translocation of food through the phloem. They supplied CO2 for photosynthesis in the bean plant, in which the carbon used was C14. As a result, the carbohydrate food produced by bean plants was also radioactive.
By autoradiography, they traced the translocation path of this food through phloem. This experiment clearly proved that food is translocated downward through sieve tubes of phloem tissue.
Composition of phloem sap
- The main component of phloem sap is sucrose. Besides, in some plants carbohydrates like raffinose, stachyose, oligosaccharide, mannitol, sorbitol, etc., are also found. Apart from these, glucose and fructose are commonly found in the phloem sap of all plants.
- Phloem sap contains amino acids like glutamic acid, aspartic acid, leucin, valine, phenylalanine, threonine, etc.
- Phloem sap also contains enzymes required in various processes of respiration.
- It also contains ions like K+, Mg2+, Cl2- PO43–, etc.
- It is composed of hormones (IAA, GA, ABA), various organic acids (mainly malic acid), etc.
Process of translocation of solute through phloem
There are various theories regarding the translocation of phloem sap through sieve tubes. Some of the theories are described below.
Munch’s mass flow or pressure flow hypothesis: This theory was first proposed by Hartig (1860). Later Munch (1930) gave its proper scientific explanation.
Munch carried out a simple experiment. He took two membranous spheres, both connected by a glass tube. The two spheres (A and B) are kept in two beakers filled with water. A contains a sugar solution and B contains water.
It would be observed that due to high osmotic pressure in A, endosmosis of water will take place. This will increase the turgor pressure in A. This will cause sugar from A to move into B. This process will continue till both the solutions attain the same osmotic pressure. If sugar is removed from B, then the flow of sugar from A will continue.
According to Munch, during the day, food is synthesized which increases the concentration of starch in the mesophyll cells of the leaf. This in turn causes an influx of water from surrounding cells to mesophyll cells and increases their turgor pressure.
The parts or organs where food is prepared in plants, i.e., the leaves are called the source, and the other nongreen portions of the plants that utilize or store the food are called the sink.
The two are connected by sieve tubes of phloem tissue through which mass flow of phloem sap occurs. In the experiment, A is the source and B is the sink and the glass tube is the comparable to sieve tube.
Evidence in support of this theory:
- Munch (1930), Dixon (1933), Huber (1941), and Crafts (1939) observed the exudation of sap by cutting phloem or through any wounds of woody stem and herbs.
- Benette (1937) and Rohrbaugh and Rice (1949) have applied C14 lebelled 2,4-D (2,4-dichlorophenoxyacetic acid, also known as chemically prepared auxin) on plants to observe the translocation of food.
Criticism against this theory:
- According to the mass flow hypothesis, the turgor pressure of the source should always be greater. But, Curtis and Schofield (1933) have experimentally proved that in most plants, a source has low turgor pressure.
- Munch stated that sieve tubes are dead cells so mass flow of sap is a passive transport. However, recent experiments proved that mass flow of sap is an active process that requires ATP.
- Munch’s hypothesis cannot explain the bidirectional translocation of sap.
Phloem loading and unloading
The translocation of food from leaves to sieve tubes is called phloem loading. Translocation of food from sieve tubes to non-photosynthetic parts of plants such as roots, stems, etc., is called phloem unloading.
Protoplasmic streaming theory:
de Vries (1855) stated that solute in phloem can move upward, due to cyclosis in protoplasm. Later, this hypothesis was explained by Curtis (1935).
Factors controlling the rate of translocation
Translocation is controlled by two groups of factors.
External factors: The external factors controlling translocation are—
Temperature: An increase in the temperature of soil increases the rate of translocation of solute towards roots. This results in a fall in the rate of movement of solutes inside the leaves as well. Generally, 25°C to 30°C is the optimum temperature range in which plants show the highest rate of translocation.
Light: A decrease in light intensity causes a decrease in the rate of translocation of solute.
Oxygen: As translocation of food is an active mechanism, 02 is required. In higher concentrations of oxygen, the rate of translocation increases.
Moisture stress: Moisture stress in leaves affects the rate of translocation.
Infernal factors: The internal factors controlling translocation are—
Solute potential: During the day, water in leaves decreases. This increases solute concentration which in turn increases the rate of translocation. The reverse occurs at night.
Translocation of sucrose through the apoplast pathway: When the concentration of sucrose increases in the apoplast pathway, the rate of translocation of solute also increases.
Number of P-proteins: P-protein is an important factor in the translocation of solutes. When P-protein gets stored in sieve tubes, it blocks the sieve plate and prevents translocation. Thus, an increase in P-protein results in a decrease in the rate of translocation.
Metabolic energy: ATP is required for translocation of solute which is essential for phloem loading and unloading. So, the rate of translocation increases with the increased rate of metabolic energy production through respiration (increased ATP production).
Turgor pressure: Recently it has been observed that water from the xylem vessel enters cells of sieve tubes and increases its turgor pressure. In comparison to the upper parts, turgor pressure in the lower parts of plants is less. This causes a downward translocation of sap. With the increase of difference of turgor pressure between the source and sink, the rate of translocation also increases.
Carrier protein: The plasma membrane of cells of sieve tubes and accessory cells contain carrier proteins. Carrier proteins like SUT1, SUT2, and SUT4 are present in the plasma membrane of cells of sieve tubes. They play an important role in the translocation of solutes.
Notes
Adhesion: The attraction between the molecules of dissimilar substances.
Apoplast: The space outside the plasma membrane within which materials can diffuse freely. It consists of intercellular spaces and xylem channels.
Bidirectional translocation: Two streams of sugars moving in opposite directions through phloem in plants.
Capillary water: The water held between the capillary spaces between the soil particles against the force of gravity.
Casparian strips: These are bands of cell wall materials mainly suberin and sometimes lignin, present in the radial and transverse walls of the endodermis.
Cohesion: The attraction between the molecules of the same substances.
Cyclosis: Cytoplasmic streaming in living cell.
Density: Mass of a substance per unit volume of it.
Eosin: A fluorescent red dye with chemical formula C2oH8Br405. Its sodium or potassium salt is used as a biological stain for cytoplasmic structures.
Gradient: An increase or decrease in the magnitude of a property, such as—temperature, pressure, concentration, etc.
Manometer: An instrument for measuring the pressure acting on a column of fluid.
Microfibril: A small fibril in the cytoplasm or cell wall, consisting of glycoproteins and cellulose.
P-protein: A phloem-specific protein found in sieve tubes in large amounts.
Plasmodesmata: A narrow thread of cytoplasm that passes through the cell walls of adjacent plant cells and allows communication between them.
Primordial utricle: The cytoplasmic lining formed on the. the inner side of the cell wall in a fully developed vacuolated cell.
Symplast: A continuous network of interconnected plant cell protoplasts in which water and low molecular weight solutes can freely diffuse.
Tensile strength: Resistance of a substance or material to breaking down under tension.
Tonoplast: A membrane that bounds the chief vacuole of a plant cell.